Correction on changing fluid TYPES
It is my understanding as follows....
If you have just bought a car and cannot verify that the transmission was serviced regularly, it is wise to stick with a good grade of regular ATF rather than switching to SYNTHETIC ATF. This is because the synthetic may loosen particles and they will act as an abrasive inside the tranny. Otherwise, it's definitely a good idea to change your trans fluid every 30K. Even with synthetics. But beware, as with synthetic engine oil, if you switch to a synthetic ATF, you may develop leaks. Just my $0.02.